There are times when it is desirable to impart heat from ambient air to a relatively cool liquid to “heat” or cool the liquid. This circumstance can arrive with respect to liquefied natural gas.
The cryogenic liquefaction of natural gas is routinely practiced as a means for converting natural gas into a more convenient form for transportation. Such liquefaction typically reduces the volume by about 600 fold and results in an end product that can be stored and transported more easily. Also, it is desirable to store excess natural gas so that it may be easily and efficiently supplied when the demand for natural gas increases. One practical means for transporting natural gas and also for storing excess natural gas is to convert the natural gas to a liquefied state for storage and/or transportation and then vaporize the liquid, as demand requires.
Natural gas often is available in areas remote from where it will ultimately be used, therefore the liquefaction of natural gas is even of greater importance. Typically, natural gas is transported via pipeline from the supply source directly to the user market. However, it has become more common that the natural gas be transported from a supply source which is separated by great distances from the user market, where a pipeline is either not available or is impractical. This is particularly true of marine transportation where transport must be made by ocean-going vessels. Ship transportation of natural gas in the gaseous state is generally not practical because of the great volume of the gas in the gaseous state, and because appreciable pressurization is required to significantly reduce the volume of the gas. Therefore, in order to store and transport natural gas, the volume of the gas is typically reduced by cooling the gas to approximately −240° F. to approximately −260° F. At this temperature, the natural gas is converted into liquefied natural gas (LNG), which possesses near atmospheric vapor pressure. Upon completion of transportation and/or storage of the LNG, the LNG must be returned to the gaseous state prior to providing the natural gas to the end user for consumption.
Typically, the re-gasification or vaporization of LNG is achieved through the employment of various heat transfer fluids, systems and processes. For example, some processes used in the art utilize evaporators that employ hot water or steam to heat the LNG to vaporize it. These heating processes have drawbacks however because the hot water or steam oftentimes freezes due to the extreme cold temperatures of the LNG which in turn causes the evaporators to clog. In order to overcome this drawback, alternative evaporators are presently used in the art, such as open rack evaporators, intermediate fluid evaporators and submerged combustion evaporators.
Open rack evaporators typically use sea water or like as a heat source for countercurrent heat exchange with LNG. Similar to the evaporators mentioned above, open rack evaporators tend to “ice up” on the evaporator surface, causing increased resistance to heat transfer. Therefore, open rack evaporators must be designed having evaporators with increased heat transfer area, which entails a higher equipment cost and increased foot print of the evaporator.
Instead of vaporizing LNG by direct heating by water or steam, as described above, evaporators of the intermediate type employ an intermediate fluid or refrigerant such as propane, fluorinated hydrocarbons or the like, having a low freezing point. The refrigerant can be heated with hot water or steam, and then the heated refrigerant or refrigerant mixture is passed through the evaporator and used to vaporize the LNG. Evaporators of this type overcome the icing and freezing episodes that are common in the previously described evaporators, however these intermediate fluid evaporators require a means for heating the refrigerant, such as a boiler or heater. These types of evaporators also have drawbacks because they are very costly to operate due to the fuel consumption of the heating means used to heat the refrigerant.
One practice currently employed in the art to overcome the high cost of operating boilers or heaters is the use of water towers, by themselves or in combination with the heaters or boilers, to heat the refrigerant that acts to vaporize the LNG. In these systems, water is passed into a water tower wherein the temperature of the water is elevated. The elevated temperature water is then used to heat the refrigerant such as glycol via a first evaporator, which in turn is used to vaporize the LNG via a second evaporator. These systems also have drawbacks however in terms of the buoyancy differential between the tower inlet steam and the tower outlet steam. The heating towers discharge large quantities of cold moist air or effluent that is very heavy compared to the ambient air. Once the cold effluent is discharged from the tower, it tends to want to sink or travel to ground because it is so much heavier than the ambient air. The cold effluent is then drawn into the water tower, hindering the heat exchange properties of the tower and causing tower to be inefficient. The aforementioned buoyancy problem causes the recirculation of cold air through water towers, hindering their ability to heat the water and essentially limiting the effectiveness of the towers.
Accordingly, there is a need in the art to provide an improved apparatus and method for imparting heat to a circulating fluid by a heating tower apparatus. It is desirable to have such apparatus and method to accomplish the vaporization of LNG that in an efficient and cost effective manner. Furthermore, there is a need in the art to provide a heating tower for use in the LNG vaporization process and/or in a vaporization system that enables the process and/or system to effectively heat water and enable the process to be more efficient and cost effective.